An MRI apparatus images an object by irradiating a high-frequency signal that is an electromagnetic wave to the object placed in a homogeneous static magnetic field generated by a static magnetic field magnet in order to nuclear spins in the object and receiving a nuclear magnetic resonance signal that is an electromagnetic wave generated by the nuclear spins in order to process the signal. The high-frequency signal irradiation and nuclear magnetic resonance signal reception are performed an RF antenna that transmits or receives radio-frequency (RF) electromagnetic waves or an antenna device referred to as an RF coil (hereinafter, referred to as an RF antenna). Actual imaging requires a static magnetic field magnet, an RF antenna, and a gradient magnetic field coil that generates a spatially gradient magnetic field. The static magnetic field magnet, the gradient magnetic field coil, and the RF antenna have a cylindrical shape respectively in a tunnel-type MRI apparatus and are normally arranged in the above order from the outer side to the innermost side i.e., the object side.
A representative RF antenna used in the tunnel-type MRI apparatus is referred to as a cylindrical volume antenna. There are various types of cylindrical volume antennas referred to as a birdcage type, a TEM (Transverse Electromagnetic) type, and a microstrip-line type (for example, refer to PTL 1 and PTL 2). In such RF antennas, normally, approximately 16 to 32 bar-shaped conductors (rung conductors) referred to as rungs (crossbars or bars of a ladder) are arranged along the side surface and parallel to the central axis of the cylinder.
The TEM-type antenna and the microstrip-line-type antenna (hereinafter, there is also a case where they are generically referred to as a TEM-type antenna) are comprised of an inner cylindrical surface and an outer cylindrical surface, the rung conductors are attached to the inner cylindrical surface, and a shield conductor functioning as an installation surface is attached to the outer cylindrical surface. Then, each of the approximately 16 to 32 rung conductors described above and the shield conductor are electrically connected and used by coupling electric currents flowing in each rung conductor with each other.